Internal combustion engines, including diesel engines, gasoline engines, natural gas engines, and other engines known in the art exhaust a complex mixture of chemical compounds. The chemical compounds may be composed of gaseous compounds, which may include nitrogen oxides (NOx), and solid particulate matter, which may include soot. Due to increased attention on the environment, exhaust emission standards have become more stringent, and the amount of pollutants emitted to the atmosphere from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine.
One method that has been implemented by engine manufacturers to comply with the regulation of particulate matter exhausted to the environment has been to remove the matter from the exhaust flow of an engine with particulate filters. To comply with the regulation of gaseous compounds, manufacturers have included various catalysts within the filters to purify the exhaust gas from the engine before emitting the gas to the atmosphere. These catalysts convert harmful NOx into innocuous constituents such as elemental nitrogen (N2) and oxygen (O2). The catalysts include base metal oxides, molten salts, and/or precious metals.
One method of providing a catalyzed particulate filter is disclosed in U.S. Pat. No. 4,912,077 (the '077 patent), issued to Lachman et al. on Mar. 27, 1990. The '077 patent incorporates catalytic materials into the filter by constructing a composite structure consisting of a catalytically active precious metal and/or base metal that becomes catalytically active upon oxidation. The structure of the '077 patent is fabricated by mixing catalytically active and structural powders with binders to form a plastic mass. The method further includes shaping the plastic mass into a desired geometry, drying the mass to remove the water content, and sintering to form a unitary composite body. The catalytically active materials of the '077 patent include chromium, cobalt, copper, iron, manganese, molybdenum, nickel, niobium, tantalum, titanium, tungsten, vanadium, zinc, zirconium, gold, iridium, osmium, palladium, platinum, rhenium, rhodium, ruthenium, and silver.
Although the method of the '077 patent may provide a catalytically active structure capable of decomposing NOx gases, the composite structure may have a low porosity, resulting in a high pressure drop across the filter, decreased filtering efficiency, and decreased engine performance. In addition, some of the materials of the '077 patent may require high activation temperatures, resulting in inefficient decomposition of NOx gases. Furthermore, the catalytic materials of the '077 patent may be costly and/or unsuited to the high temperature environment of a particulate filter.
The disclosed exhaust system is directed to overcoming one or more of the problems set forth above.